Method of producing electron multiplication utilizing an amplification cycle



March 21, 1967 J. c. KYLANDER ETAL 3,310,673

METHOD OF PRODUCING ELECTRON MULTIPLICATION UTILIZING AN AMPLIFICATION CYCLE 2Sheets-Sheet 2 Filed June 5, 1964 M M a 7 AC .H a y M W P II I l 4 4 i J w W W. w 4 U m/ a H w z a ww r, 4 W M A 6 z r f i Z W llll M M 1 w P M 0 P a 7%% /%%J7// 5 mm: PM NW0 f 2 ,m l 4 mm 6 M M 0 MW.

United States Patent 3,310,678 METHOD OF PRODUCING ELECTRON MUL- TIPLICATIQN UTILIZING AN AMPLIFlCA- TION CYCLE John C. Kylander, Fort Wayne, Ind., and William J.

Soule, Brookline, Mass, assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Air Force Filed June 5, 1964, Ser. No. 373,141 1 Claim. (Cl. 250-207) This invention relates to a photomultiplier and, more particularly, to a photomultiplier structure and a method of producing electron multiplication utilizing a transmissive type secondary electron multiplier.

In the prior art of photomultiplication, electrons emitted from a photocathode as a result of incident radiation are amplified by secondary emission. The electrode i11- corporated in a photomultiplier which undergoes aforesaid secondary emission upon bombardment by photoelectrons is generally referred to as the dynode. A photomultiplier may have one or more dynodes. Generally, the prior art photomultiplication was achieved by impressing or alternating voltage between the dynodes until a desired output amplitude was attained. However, there were limitations, for example, the increase of size in the photocathode introduced spread in transit time for electrons moving from different areas of the photocathode to the front dynode; there was an inefficient collection of electrons and there was a problem introduced because of ion feedback.

In accordance with the present invention a method and structure for photomultiplication is provided wherein a pair of dynodes are included. An anode is interposed between the dynodes. A duty cycle controlling grid is disposed between a photoemissive cathode and one of aforesaid dynodes. The control grid is used to provide a means of pulsing the photocathode current off and on in a manner such that during the amplification cycle no photocurrent is emitted at the cathode. Also, the control grid can be used as a means of controlling the transit time of electrons going from the photocathode to one of the dynodes. Thus the aforementioned limitations of the prior art are eliminated or improved.

An object of the present invention ,is to provide method and structure for photomultiplication wherein electrons are efficiently collected.

Another object of the present invention is to provide method and structure for photomultiplication wherein no photocurrent is emitted at the cathode during the amplification cycle.

Yet another object of the present invention is to provide a method and structure for photomultiplication wherein ion feedback is eliminated.

The various features of novelty which characterize this invention are pointed out with particularity in the claim annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, ref erence should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

FIG. 1 is a diagrammatic view of an electron multiplier tube embodying our invention;

FIG. 2 shows the front surface secondary emission yield curve of a dynode electrode incorporated in the electron multiplier tube illustrated in FIG. 1; and

FIG. 3 includes the electron multiplier tube of FIG. 1 and the means for providing the operating potentials therefor.

Now referring in detail to FIG. 1, there is shown an electron discharge device tube which embodies an elec- 3,319,678 Patented Mar. 21, 1967 tron multiplying structure. The tube is comprised of glass envelope 10. Photoemissive photocathode 11 is deposited on the interior of glass faceplate 12 and may be substantially planar. Photocathode 11 may be of the type shown and described in US. Patent No. 2,617,948, issued Nov. 14, 1952. Control grid 13 is positioned adjacent to photocathode 11 and may be in the form of a mesh. Adjacent to control grid 13 is positioned dynode 14 which is a transmittive type secondary electron dynode, consisting basically of a thin film of metal followed by a film of an insulator. Anode 15 is a high transmission mesh and is adjacent to dynode 14. -Dynode 1 6 is a metal plate treated such as to have good secondary emission properties. Dynodes 14 and 16 may be of the type shown and described in US. Patent Nos. 2,898,499, issued Aug. 4, 1959, and 2,617,948, issued Nov. 11, 1952. Control grid 13, a mesh, is interposed between photocathode 11 and dynode 14 for two purposes. This mesh is used to provide a means of pulsing the photocathode current off and on in a manner such that during the amplification cycle, to be discussed later, no photocurrent is emitted at photocathode 11. Voltage sources 21 and 22 of FIG. 3 are connected to provide the conventional operating potentials between electrodes 11, 13, 14, and 1 6. Also, this mesh can be used as a means of controlling the transit time of the electrons going from the photocathode 11 to dynode 14. Focus coil 17 provides a magnetic field to focus both primary and secondary electrons. It is to be noted that electrodes 11, 13, 14, 15, and 16 may be planar and so arranged and positioned to be of plane'- parallel construction.

Photoelectrons are emitted in direct proportion to the light intensity which reaches photocathode 11 by way of glass faceplate 12, the light entrance aperture. These photoelectrons are accelerated and magnetically focused onto dynode 14. The photoelectrons bombard thin film dynode 14 with sufiiciently high energy to penetrate the film and cause secondary electrons to be emitted from the back side. On the exit side of this film, there is a secondary emitting surface. The primary to secondary electron ratio at the exit side of the film is kept low by having a low value of primary voltage.

Between dynodes 14 and 16, an alternating voltage supplied by alternating voltage generator 20 of FIG. 3 is applied which accelerates the secondary electrons emitted from the exit side of dynode 14 onto dynode 16, the voltage polarity is reversed as secondary electrons are released from dynode 16, and these electrons emitted from dynode 16 are accelerated back to dynode 14. Again the alternating voltage polarity is changed as the secondary electrons are emitted from dynode 14 so that these electrons return to dynode 16. This amplification cycle, that is the period during which electron multiplication is taking place between dynodes 14 and 16, continues until the desired signal output amplitude is attained. The duty cycle at which the alternating voltage polarity is reversed will depend on the transit time between dynodes 14 and 16.

The secondary electrons are focused from one dynode to the other by means of external focus coil 17. The secondary electrons emitted from dynode 14 are accelerated to dynode 16 with sufiicient energy to obtain a high secondary emission ratio. Likewise the secondary electrons emitted at dynode 16 are accelerated to dynode 14 with only enough energy to cause front surface secondary emission.

At the appropriate time during the amplification cycle as determined by conventional timing means 25 of FIG. 3 (depending upon the magnitude of amplification desired) anode mesh 15 is voltage pulsed by means of positive pulse generator 26 of FIG. 3 thereby attaining eflicient collection of the amplified current. The potential and transmission of the anode is such, as provided by voltage source 23 of FIG. 3, that during the amplification cycle the amount of current intercepted by the mesh structure is small, thus, there is no appreciable reduction in the total output signal. Only during the off period of the amplification cycle is photocurrent permitted to flow between photocathode 11 and dynode 14. The aforementioned 01f period of the amplification cycle is provided by applying a negative pulse to central grid 13 from negative pulse generator 24 of FIG. 3. It is noted that the appropriate timing for pulse generators 24 and 26 of FIG. 3 is provided by any conventional timing means 25 of FIG. 3.

The thin film dynode 14 structure is similar to films described in Research Report 6947151R1 from Westinghouse Laboratory and also in the aforementioned U.S. Patent No. 2,898,499. The front surface secondary emission yield curve of a film having a MgO surface is shown in FIG. 2. The secondary emitting surface of dynode 16 can have similar properties.

Thus the present invention provides several advantages over conventional photomultiplier structures. ample, the light entrance aperture to the multiplier structure is as large as dynode 14. Therefore, the photocathode can be very large without introducing spread in transit time for electrons moving from different areas of the photocathode to the front dynode. Due to the planeparallel construction, there is a reduction of transit time spread. Due to the opaque nature of the dynode structures, efficient collection of all electrons is possible. Since both photoelectrons and secondary electrons move along the axial direction of the structure, it is possible to eliminate effects of strong magnetic fields For ex-' by simply orienting the tube axis parallel to the magnetic lines of flux of the focus coil. Ion feedback is essentially eliminated by the fact that ions formed in the dynode 14- dynode 16 are unable to penetrate through dynode 14.

A further application of this multiplication technique could be used as an amplifier section in the image orthicon.

What we claim is:

A method for amplifying electron current resulting from light incident upon a photoemissive surface comprising channeling the current provided by light incident upon a photoemissive surface to a first secondary emitter to provide secondary emission therefrom, directing said secondary emission from said first emitter upon a second secondary emitter to further provide secondary emission therefrom, re-directing said secondary emission from said second emitter back to said first emitter, alternating said directing and re-directing for a preselected period of time in accordance with the requisite amplification of current, thus providing an amplification cycle and switching said current from said photoemissive surface off and on at predetermined times so that no current is emitted from said photoemissive surface during said amplification cycle.

References Cited by the Examiner UNITED STATES PATENTS 2,227,103 12/ 1940 Orthuber et al. 313-104 2,617,948 11/ 1952 Kallmann 250207 3,229,213 1/ 1966 Schwartz 3 13104 JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner. 

